1. Technical Field
The present invention is directed to a metal coating coupling composition. More particularly, the invention is directed to a metal coating coupling composition, which is composed of (A) a silane cross-linking agent and (B) a silane coupling agent and applied in metal coating systems to enhance adhesion and improve corrosion resistance.
2. Background Art
A metal coating system is a chemical and/or physical joint system that includes a metallic substrate coated by a polymeric coating. A metallic substrate as hereinafter referred to includes all metallic surfaces of matter, such as ferrous metals, nonferrous metals, ferrous/nonferrous metal-coated items, ferrous/nonferrous alloys, ferrous/nonferrous alloy-coated items and the like. A polymeric coating as hereinafter referred to includes all organic coatings, such as, paint, lacquer, varnish, synthetic resin, ink, enamel and the like. The performance of a metal coating system is generally desired for longer service life and normally depends, besides upon quality of the substrate and the coating, significantly upon adhesion across each interface and upon cohesion across each layer in the metal coating system as well. A good corrosion resistance of a metal coating system results from the establishment of good adhesion and cohesion in such systems; whereas poor adhesion and/or cohesion usually cause coating delamination even blistering, which substantially diminishes the protection of a metal by a polymeric coating in a metal coating systems.
It is well known that a polymeric coating generally does not adhere well onto a metallic substrate due to incompatibilities of one another, additionally due to cleanliness of the substrates. Therefore, a metallic surface has to be cleaned and/or treated prior to applying a polymeric coating on it. In general, a metallic surface is cleaned first to remove any soils including oil, grease, metalworking fluid, fingerprint oil, lubricant, rust, scale, etc. Then, a conversion coating is applied to improve the compatibilities between two types of materials. As a result, the improvement of compatibilities enhances adhesion in a metal coating system and prolongs the service life of such system. Coatings may be of two types here, 1) phosphate conversion coatings and 2) primer coatings. A phosphate conversion coating, including iron and zinc phosphate coating, physically improves adhesion in a metal coating system; whereas, a primer coating chemically improves adhesion in a metal coating system. A primer is a type of adhesive, which includes primer, glue, gum, starch, and size, etc., termed upon the thickness of adhesive applied in a joint system. In general, the thickness of an adhesive layer applied on a metallic substrate in a metal coating system is within a magnitude of molecular level, which is commonly classified as a primer. The metal coating coupling composition of this invention is a primer used in metal coating systems, and, also, alternatively termed as “final seal”, “final rinse”, “passivating rinse”, “post rinse”, or “post-treatment” in the metal finishing industry.
It is also well known to employ a chromium-type primer, typically chromate and chromium phosphate, in a metal coating system to enhance adhesion and improve corrosion resistance. Chromium, a diverse element, demonstrates impressively acceptable performance and effectiveness in varieties of combination of metal coating systems including in conjunction with iron or zincphosphate conversion coating in terms of different types of metals or their alloys and different types of polymeric coatings. Accordingly, the chromate primer is widely considered and universally applicable in metal coating systems. In addition, the chromate treatment has the advantages of excellently anti-corrosive characteristics, and the use of chromate is relatively simple and inexpensive. Unfortunately, serious concerns of use of chromate primer and disposal of waste chromate solution have been raised regarding their negative effects to environment and human beings. Because chromate, specifically hexacovalent chromate, is toxic to the environment, carcinogenic to humans, and very difficult to recycle, even with extensive waste treatment procedures, as a result, considerable expense is contributed for handling, applying, and hazardous waste disposing. The eliminating or minimizing the use of the chromate primer had been an objective for a number of years. In the past, extensive research has been contributed to develop chromate-free type of primers used in metal coating systems, which may generally be classified, based upon the type of compound used, as silane coupling agents, polymeric materials, metallic compounds, and mixtures of them. In general, chromate-free metal coating coupling composition taught by previous arts for metal coating applications may be classified as following types based on chemical compounds:
1. Arts of Polymer Types    U.S. Pat. Nos. 4,433,015, 4,517,028, 4,795,506, 4,963,596, 5,039,770, 5,063,089, 5,868,820, 5,961,809, 4,191,596, 5,282,905, 5,641,542, 5,753,779, 5,433,773, and 5,472,523.
2. Arts of Silane/Silicate Types of Metal Coating Couplings    U.S. Pat. Nos. 5,108,793, 5,200,275, 5,700,523, 5,433,976, 5,478,655, 5,292,549, 6,071,566, 6,106,901, 6,132,808, 5,750,197, 5,759,629, 5,393,353, 6,084,106, 4,812,363, and 5,882,799.
3. Arts of Metallic Element Types    U.S. Pat. Nos. 1,710,743, 3,682,713, 3,695,942, 3,895,970, 4,148,670, 5,209,788, 5,868,872, 5,885,373, 6,027,579, 5,843,242, and 5,993,567.
4. Arts of Polymer/Silane Types    U.S. Pat. Nos. 5,612,421, and 5,846,342.
5. Arts of Polymer/Metallic Element Types    U.S. Pat. Nos. 5,859,106, 5,859,107, 5,662,746, 5,859,107, and 5,801,217.
6. Arts of Silane/Metallic Element Types    U.S. Pat. Nos. 5,026,440, 5,397,390, and 5,518,770.
7. Arts of Miscellaneous Types    U.S. Pat. Nos. 2,854,368, 3,975,214, 4,944,812, 5,868,820, 3,547,710, 5,128,211, 5,607,521, and 5,700,523.
Arts pertinent to metal coating coupling compositions used for metal coating applications cited hereinbefore indicate extensive and intensive researches have been conducted in the past, but, so far, the replacement of chromate primer for metal coating applications is not fully satisfactory. Summarily, each aforementioned have at least one of following limitations:
Inconsistent performance in term of adhesion and corrosion resistance for different types of polymeric coatings;
Costly operation, including high concentration of bath solution (up to 10% of active chemicals in working tank), expensive synthetics (some polymeric metal coating couplings), multi-package of products, and multi-step processes;
expensive manufacturing, such as some compositions requires days to hydrolyze and some remain significant unreacted raw materials;
Staining or discoloring metallic substrates;
High content of volatile organic compound (VOC);
Containing Heavy metal.
Among silane coupling agents, polymeric materials, metallic compounds and based upon the natural characteristics of metal coating systems, it is believed that highly polarized silane potentially contribute to form bonds, maybe covalent bonds, between a metal substrate and a coating in a metal coating system. A silane composition may function the same as chromate metal coating coupling does in metal coating systems to covalently bond a metal and a coating. Application of silane has prevalent advantages as a primer in metal coating system, for instance, inexpensive product, operable manufacturing, and no further environmental issues. Since silanes widely contribute to various types of organofunctional groups reactive to polymeric coatings and hydrolyzable groups reactive to metallic surface, silanes may contribute to enhance adhesion in a metal coating system specially and may replace iron-phosphatizing stage in phosphate processes.
It is an object of the present invention to provide a method to optimize or maximize performance to enhance adhesion and improve corrosion resistance in metal coating systems. Particularly, it is an object of the present invention to provide a selected-silane compound to apply in a specific metal coating system, which involves a polyester coating or its hybrid coating.
It is further object of the present invention to provide a metal coating coupling composition to enhance adhesion and improve corrosion resistance with or without conventional conversion coating, such as iron-phosphate conversion coatings, in metal coating systems.
Particularly, it is an object to apply an aqueous metal coating coupling composition on a bare metal surface, subsequently coated with a polyester coating or its hybrid coating to replace iron-phosphatizing, iron-phosphatizing/chromating process, or chromating involved in zinc-phosphatizing/chromating process without lowering the performance of such metal coating system. Replacements are illustrated as follows, respectively:
TABLE 1Replacement of Iron-phosphatizing/chromating ProcessProposed ProcessConventional ProcessPreparation (s)*Preparation(s)*Application of ProposedIron PhosphatizingMetal coating couplingWater RinsingcompositionChromate RinsingWater Rinsing**DryingDryingPolymeric CoatingPolymeric Coating*Preparation(s) process may include degreasing and/or descaling stages, water rinsings as well. **The process of exclusion of this stage after post-treatment, that is, no additional rinsing is need after applying of the metal coating coupling of this invention, is generally termed as “dried-in-place” technology. 
TABLE 2Replacement of Chromating in Zinc-Phosphatizing ProcessProposed ProcessConventional ProcessPreparation (s)*Preparation(s)*Zinc PhosphatizingZinc PhosphatizingWater RinsingWater RinsingProposed Metal coatingChromate Rinsingcoupling compositionWater Rinsing**DryingDryingPolymeric CoatingPolymeric Coating*Preparation(s) process may include degreasing and/or descaling stages, water rinsings as well. **The process of exclusion of this stage after post-treatment, that is, no additional rinsing is need after applying of the metal coating coupling of this invention, is generally termed as “dried-in-place” technology. 
It is another object of the present invention to provide an aqueous metal coating coupling composition, which works on bare metal substrate to replace chromate metal coating coupling compositions without lowering the performance in metal coating applications. The embodiment of this object is to provide an aqueous solution to be directly applied on clean metal surface, dry in place, and apply a polyester coating or its hybrid coating on, without lowering the performance of metal coating systems.